An example of the general kind of apparatus to which this invention relates is disclosed in U.S. Pat. No. 3,788,382. In the apparatus disclosed in this patent, a metal is electrically heated in a crucible to optimum casting temperature. The temperature of the molten metal is sensed and, in response to such sensing, the molten metal is sucked under vacuum into a mold through an opening provided in the bottom of the crucible, with the vacuum being produced in the mold. In practice, apparatus such as that shown in the noted patent tends to have certain disadvantages, among which are the following:
(a) The products made with such apparatus are prone to having faults therein. The reasons for such faults can be traced to a number of factors: (1) the range of the optimum casting temperatures is unfavorably limited; (2) the detector of the optimum temperature may fail to detect an average temperature of the whole body of the molten metal, in which temperature can differ from spot to spot, thereby resulting in an improper detection of the temperature so that the molten metal is sucked into the mold either too soon or too late; and (3) there is inherent in the arrangement an unavoidable time lag between the detection of the optimum temperature and the initiation of the suction vacuum, during which time lag the molten metal is in danger either of excessive heating or detrimental cooling. When excessively heated, the metal is liable to form rough surfaces, blowholes and pinholes. When cooled below the optimum casting temperature the molten metal lacks its optimum fluidity and, for this reason, can fail to reach deeply enough into molding cavities. Accordingly, when the temperature of the molten metal is not optimum, it sometimes happens that a particular desired shape or surface condition for an object cannot be achieved, and other related problems can be encountered.
(b) The melting temperature and the optimum casting temperature are different, depending upon the kind of metal. When using apparatus like that shown in the patent, a complicated procedure is required for predetermining the optimum melting temperature.
(c) A temperature detector device and an electronic control device connected with the detector and arranged to operate a vacuum pump in response to the detection of a specific temperature by the detector are required in apparatus like that shown in the patent. These devices require a high degree of reliability and, with their inclusion, the apparatus as a whole becomes both expensive and complicated.
(d) In apparatus like that shown in the patent, the reliability of the temperature detector device and of the electronic control device must be maintained, requiring a constantly ongoing, labor-consuming maintenance effort. But even with such a maintenance effort, the life of these devices tends to be short.
(e) In order to perform the resistance heating of a metal in the apparatus of the patent, a vertically split type of crucible is employed, with an insulating plate interposed between its paired sides. In addition, a backing means must be provided for securing the metal in between the paired sides of the crucible. This arrangement again requires expensive maintenance, and makes the apparatus structurally complicated and costly.